It is well known in the paper industry that a wide variety of fillers and pigments such as titanium dioxide, calcium carbonate, silica, alumina and kaolin, are suitable for use as paper fillers. Kaolin, a hydrous aluminum silicate, is presently the most widely utilized of these and is available in a range of particle sizes and brightnesses, as well as being either delaminated or non-delaminated. Hydrous kaolin is white in color, has a fine particle size, and is relatively chemically inert. This, in addition to its low cost makes it an ideal paper filler. Although calcined (anhydrous) kaolin is also available for use as a paper filler and can impart greater opacity to paper than the hydrous kaolin, it has the serious disadvantage of generally being more abrasive.
Prior art kaolin paper fillers are typically produced by a beneficiation process which typically consists of fractionating in a continuous centrifuge to controlled particle size, followed by bleaching to remove iron-based colored compounds. In the bleaching process the kaolin is acidified with H.sub.2 SO.sub.4 to a pH of about 3.0, and sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The flocculated clay, generally at approximately 30% solids by weight, is then filtered, such as by dewatering on a rotary vacuum filter to a solids level of approximately 60% by weight. The filter cake is then either dried or redispersed with additional dry clay if it is to be sold as approximately 70% by weight solids slurry. To produce high brightness products, i.e. fillers having a brightness index greater than 90 (as measured by TAPPI procedure T-646-os-75), impurities may be removed from the kaolin clay by further processing the kaolin clay through flotation or magnetic separation. To produce a delaminated product, the coarse fraction from the initial centrifugation is ground in sand grinders to shear the stacks of platelets normally found in kaolin and thereby produce individual particles having an equivalent spherical diameter less than 2 microns.
The anhydrous kaolin products generally available as paper fillers are typically produced by calcining hydrous kaolin at temperatures up to 1050.degree. C. so that structural hydroxyl groups are driven out as water vapor. The resulting material has an amorphous structure which contains voids which produce interfaces between kaolin and air. These interfaces of kaolin and air, which are not found in hydrated kaolin, serve as sites for light scattering. Because of these voids, calcined clay has greater optical efficiency than other kaolin fillers.
Other opacifying pigments are commercially available to the papermaker. Because of its high refractive index, 2.55 for anatase and 2.7 for rutile, titanium dioxide is presently the opacifier of primary commercial importance. When incorporated into paper, titanium dioxide also imparts exceptional brightness and whiteness to the sheet. However, the main disadvantage of titania is its cost. Commercial grade titania is approximately 4 times more costly than commercial grade anhydrous kaolin and up to 25 times higher than the commercial grade hydrous kaolin. Due to this cost factor, other products have been developed and are in commercial use as titania "extender" pigments. These products, which can be used to replace portions of the titanium dioxide without a loss of opacity of the paper, include calcined clay, delaminated hydrous clays, fine particle size silica and alumina, and sodium aluminum silicate. The effectiveness of either calcined or delaminated kaolin clay as extenders for titanium dioxide can, in part, be attributed to the paucity of colloidal fines, i.e. particles having a fineness of less than about 0.3 microns equivalent spherical diameter. Calcined kaolin can be produced having a content of only 5 to 10% by weight of colloidal particles and a brightness of 80-93 and standard filler clay can be produced having a content as high as about 40% by weight of colloidal particles.